1. Field of the Invention
The present invention generally relates to three-terminal variable inductors, and more particularly, to a three-terminal inductor for use in, for example, mobile communication devices.
2. Description of the Related Art
It is becoming more and more critical that current electronic devices have smaller sizes, and especially for mobile communication devices such as cellular phones and automobile phones, components incorporated therein must be severely restricted in size. In addition, as the operating frequency becomes higher, the circuit and components defining circuits in such devices become more complex, and the components tolerate less variations. Conventionally, in manufacturing a circuit having a center tap connected to the electric center of a coil, a pair of coil components 101 and 102 are mounted on a printed circuit board 106, and are electrically connected to each other via electrodes 103 and 104 and a center tap electrode 105 provided on the printed circuit board 106, as shown in FIG. 9. As methods of changing the values of inductance of the coil components 101 and 102, it has been proposed, for example, that the coil components 101 and 102 be replaced by other two coil components having different values of inductance which are balanced in advance, or variable coils be used as the coil components 101 and 102 and the values of inductance thereof be changed while maintaining the balance of the inductance values.
In accordance with these methods, however, due to component variations or displacements during mounting, the inductance values of the coil components 101 and 102 may not be well balanced. It may also occur that the center tap electrode is connected off the electric center of the coil defined by the coil components 101 and 102. In addition, because the two coil components 101 and 102 are electrically connected via the center tap electrode 105 provided on the printed circuit board 106, a considerable area is occupied on the printed circuit board 106.
Furthermore, as for the method which changes the inductance values by replacing the coil components 101 and 102 with other two coil components, the complex work of removing the coil components has inhibited automation and mass production. Similarly, as for the method which uses variable coils as the coil components 101 and 102 to change the inductance values of the coil components 101 and 102, the complex work of adjusting the inductance values while maintaining the balance therebetween has also inhibited automation and mass production.
In order to overcome these problems, a three-terminal variable inductor 110 shown in FIG. 10 has been proposed. In the three-terminal variable inductor 110, a pair of spiral coil electrodes 112 and 113 having identical dimensions are disposed on the top surface of an insulative substrate 111. The spiral coil electrodes 112 and 113 are electrically connected to trimming electrodes 116a to 116d via openings provided on an insulating protective film 115. The trimming electrodes 116a to 116d are connected to a center tap electrode 117, thereby being electrically connected to a common terminal electrode 122. One end of the coil electrode 112 and one end of the coil electrode 113 are electrically connected to a terminal electrode 120 and a terminal electrode 121, respectively.
In order to adjust the value of inductance of the three-terminal variable inductor 110, the trimming electrodes 116a to 116d are cut one by one as desired, for example, by irradiating laser beams on the three-terminal variable inductor 110. Accordingly, the value of inductance between the terminal electrode 120 and the common terminal electrode 122 and the value of inductance between the terminal electrode 121 and the common terminal electrode 122 can be changed in steps while maintaining the balance therebetween.
In the three-terminal variable inductor 110, however, because the trimming electrodes 116a to 116d are arranged to partially overlap the spiral coil electrodes 112 and 113, stray capacitance between the trimming electrodes 116a to 116d and the spiral coil electrodes 112 and 113 is large. Therefore, the three-terminal variable inductor 110 has a low self resonance frequency and fails to provide favorable frequency characteristics at higher frequencies. In addition, the trimming electrodes 116a to 116d shield magnetic fields generated by the spiral coil electrodes 112 and 113, resulting in inadequate Q characteristics of the three-terminal variable inductor 110.
In order to solve the problems described above, preferred embodiments of the present invention provide a three-terminal variable inductor which is very small, minimizes the occupied area on a printed circuit board when mounted thereon, achieves stable adjustment of inductance values while maintaining a good balance, and which achieves excellent characteristics.
According to a preferred embodiment of the present invention, a three-terminal variable inductor includes a first terminal electrode, a second terminal electrode, a third terminal electrode, a first spiral coil electrode electrically connected between the first terminal electrode and the third terminal electrode, an inner end portion thereof being associated with the first terminal electrode and an outer portion thereof being associated with the third terminal electrode, a second spiral coil electrode electrically connected between the second terminal electrode and the third terminal electrode, an inner end portion thereof being associated with the second terminal electrode and an outer portion thereof being associated with the third terminal electrode, and at least one trimming electrode arranged so as not to cross any portion of the first spiral coil electrode and the second spiral coil electrode, between the outer portion of the first spiral coil electrode and the outer portion of the second spiral coil electrode, the outer portions being disposed in proximity to each other, the at least one trimming electrode electrically connecting the first spiral coil electrode and the second spiral coil electrode.
In accordance with the unique construction of the preferred embodiment of the present invention described above, by trimming the at least one trimming electrode, the value of inductance between the first terminal electrode and the second terminal electrode can be changed without disturbing the balance of the inductance value between the first terminal electrode and the third terminal electrode and the inductance value between the second terminal electrode and the third terminal electrode. In addition, since the trimming electrode is arranged so as not to cross any portion of the first spiral coil electrode and the second spiral coil electrode, the three-terminal variable inductor has a small stray capacitance between the trimming electrode and the first and second spiral coil electrodes and therefore has a high self resonance frequency, thus exhibiting favorable frequency characteristics at high frequency bands. Furthermore, since the trimming electrode does not block magnetic fields generated by the first and the second spiral coil electrodes, Q characteristics are greatly improved.
The three-terminal variable inductor may further include a plurality of trimming electrodes, and a center tap electrode electrically connected to the third terminal electrode is disposed between the outer portion of the first spiral coil electrode and the outer portion of the second spiral coil electrode, the outer portions being disposed in proximity to each other, the plurality of trimming electrodes being electrically connected to the center tap electrode.
In accordance with the above-described unique construction, by trimming the at least one trimming electrode, the value of inductance between the first and second terminal electrodes, the value of inductance between the first and third terminal electrodes, and the value of inductance between the second and third terminal electrodes can be changed without disturbing the balance of the value of inductance between the first and third terminal electrodes and the value of inductance between the second and third terminal electrodes.
The first terminal electrode, the second terminal electrode, the third terminal electrode, the first spiral coil electrode, the second spiral coil electrode, and the at least one trimming electrode may be disposed on the surface of an insulative substrate of a chip component.
Alternatively, the first terminal electrode, the second terminal electrode, the third terminal electrode, the first spiral coil electrode, the second spiral coil electrode, and the at least one trimming electrode may be disposed on the surface of a circuit board provided with a circuit pattern.
In accordance with either of the above unique constructions according to preferred embodiments of the present invention, the at least one trimming electrode and the first and the second spiral coil electrodes are disposed on a single layer, so that the number of interlayer connections is reduced, thereby the inductance component having a high reliability of the connections between layers is obtained.
Furthermore, because the trimming electrode is connected to the outermost portions of the first and the second spiral coil electrodes, the electrodes can be arranged to be substantially parallel to each other efficiently using the region in the longitudinal direction of the insulative substrate. Thus, the trimming electrodes can be disposed in an extended area, and therefore, the variable range of inductance value can be increased by approximately 10% compared with conventional variable inductors. In addition, the first and second spiral coil electrodes can also be disposed in a larger area, achieving an improvement of approximately 5% in the maximum obtainable inductance value.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments with reference to the attached drawings.